The present invention relates to a method of preparing an emulsion, more specifically to a method of efficient organic solvent removal in the production of an oil-in-water emulsion.
Oil-in-water emulsions are widely used to produce photographic light-sensitive materials, cosmetics, foods, paints, drugs and other items.
For example, in the production of a photographic light-sensitive material, an emulsion is prepared using oil-soluble substances such as a dye image forming compound (e.g., color coupler), a diffusion transfer compound, an antifogging agent, an antifading agent, a color mixing preventing agent, an ultraviolet absorbent and a color sensitizer. An oil-in-water emulsion of these oil-soluble substances is generally prepared by adding either oil-soluble substances along with an organic solvent or an emulsification aid or with an emulsification aid in solution in an organic solvent, or an oil-phase solution prepared by either heating an oil-soluble substance or dissolving it in an organic solvent when the oil-soluble substance is solid (hereinafter simply referred to as oil-phase solution) to an aqueous-phase solution containing a water-soluble binder supplemented with an emulsification aid added as necessary (hereinafter simply referred to as aqueous-phase solution) to yield an oil-in-water emulsion of about 0.1 to 1.0 .mu.m in average grain size.
The organic solvent described above is often a solvent having a boiling point below that of water, such as ethyl acetate. Since this solvent becomes unnecessary upon completion of dispersion, it is removed by, for example, reducing the pressure in the emulsification tank with a vacuum pump connected thereto.
A variety of methods of organic solvent removal are known, of which the method in which the organic solvent is distilled off under reduced pressure ranks best since the other methods have disadvantages of low efficiency or speed of solvent removal, solution denaturation, increased equipment cost, etc.
On the other hand, many of the formulations used to produce photographic light-sensitive materials show a foaming property when emulsified and dispersed.
When such a foaming hydrophilic colloid solution is emulsified and dispersed, foam fills the emulsification tank and reduces the evaporation area, thus hampering the smooth removal of the solvent. In addition, when the solvent is to be removed by distillation under reduced pressure, the foam migrates into the exhaust piping, which results in significant loss and necessitates cleaning of the inside of the exhaust piping upon every completion of emulsification dispersion. Meanwhile, Japanese Patent Examined Publication No. 56010/1986 discloses the removal of organic solvent under reduced pressure wherein a reduced pressure of about 500 Torr is maintained in the initial stage, followed by further reducing the pressure to about 30 to 150 Torr 1 to 2 minutes later to reach a stationary reduced pressure condition.
However, validation experiments conducted by the present inventors revealed that this method causes significant loss of the emulsion due to severe foaming with rapid change in pressure and requires much labor to wash down the foam migrated into the exhaust piping.